1. Field of the Invention
The invention relates to selective naphtha desulfurization with reduced mercaptan formation and olefin saturation. More particularly, the invention relates to selectively removing sulfur compounds from a sulfur and olefin-containing naphtha feed, with reduced reversion mercaptan formation and olefin saturation, by reacting the feed with a hydrogen treat gas at a high temperature, in the presence of a low metal loaded, partially deactivated and at least partially regenerable hydrodesulfurization catalyst.
2. Background of the Invention
Future mogas sulfur specifications are being regulated through legislation to increasingly lower levels, due to environmental considerations. Sulfur specifications on the order of less than 150 wppm of total sulfur are likely near term, with values of no greater than 30 wppm of total sulfur possible in the not too distant future. Such sulfur specifications are without precedent and will require the production of low sulfur blend stock for the mogas pool. The primary sulfur sources in the mogas pool are the blend stocks derived from FCC naphthas, whose sulfur content can fall in the range of 1500-7000 wppm depending upon crude quality and FCC operation. Conventional fixed bed hydrodesulfurization can reduce the sulfur level of FCC naphthas to very low levels, but the severe conditions of temperature, pressure and hydrogen treat gas ratio results in significant octane loss, due to olefin loss from saturation. Selective hydrodesulfurization processes have been developed to reduce olefin saturation and concomitant octane loss. Such processes are disclosed, for example, in U.S. Pat. Nos. 4,149,965; 5,525,211; 5,243,975, and 5,906,730, some of which employ poisoned and permanently deactivated catalysts. The process disclosed in the '975 patent employs a deactivated resid catalyst loaded with at least 1% of nickel, iron and vanadium poisons. Besides introducing possible contaminants into the naphtha stream, this catalyst is not regenerable. On the other hand, the '211 patent teaches the use of a permanently deactivated catalyst composition containing alkali or alkaline metal. The process disclosed in the '965 patent teaches that the selectivity of a naphtha hydrodesulfurization catalyst for sulfur removal slightly increases over time. Catalyst deactivations of from 2 to 50% are disclosed. In these and in other processes, in the hydrodesulfurization reactor the H.sub.2 S formed as a consequence of the hydrodesulfurization reacts with the feed olefins, to form mercaptan sulfur compounds, which are known as reversion mercaptans. The amount of these mercaptans formed during the process typically exceeds future fuel specifications for mercaptan sulfur and, in some cases, total sulfur. Accordingly, it is desirable to have a deep and selective desulfurization process, with reduced mercaptan reversion and octane loss without introducing contaminants into the process and preferably with a catalyst that has not been permanently deactivated.